The quality and value of paper is directly related to the quality and uniformity of the fiber stock used to produce it. Modern sources of pulp fibers, especially fibers from recycled materials, fibers produced from tropical hardwood, and fibers produced from wood chips which have been stored in the open, are contaminated with various impurities. These impurities include lightweight particles of resin from tropical hardwood, lightweight particles of plastic and hot glue from recycled paper, broken fiber fragments from recycled paper, and heavy weight particles including sand and dirt. Hydrocyclones have found widespread use in the papermaking industry for cleaning and improving the quality of stock used for forming a paper web. Hydrocyclones employ a combination of gravity, centrifugal force, and hydrodynamic forces to separate particles and fibers of varying density and size.
Recent developments have resulted in hydrocyclones which can separate both high and low-density materials from fibers at the same time. The art related to hydrocyclones continues to develop and improve, nevertheless, it remains true that often several cleaning cycles are needed to perform an adequate separation and cleaning of a given feed of fluid containing fiber and contaminates.
Other principles for cleaning fibers are employed in other types of devices. For example, fibers are screened by forcing them to pass through screens of varying sizes. Sedimentation and flotation, including dissolved air-assisted flotation, are used in clarifying water containing fibers. Recently a new technique has utilized ultrasound to create a pressure gradient on particles which is size dependent. This techniques has been used expressly to clarify water containing pulp fibers. However these techniques have not contributed to the improvement in the design of hydrocyclones.
Additional physical forces or principles which could be employed in hydrocyclones might allow significant additional improvements in efficiency and throughput for this widely used class of devices.